[libc++abi] Make the error message for recursive initialization of function-local statics more explicit

[ldionne]

Summary

The new message is a bit verbose, but it makes it clearer that this isn't
a problem in libc++abi, but instead a problem in the user's code. Otherwise,
we get bugs sent down to libc++abi because people see this message in their
crash logs.

libcxxabi/src/cxa_guard_impl.h

struct InitByteNoThreads {
   explicit InitByteNoThreads(uint8_t* _init_byte_address, uint32_t*) : init_byte_address(_init_byte_address) {}
 
   /// The init byte portion of cxa_guard_acquire. Returns true if
   /// initialization has already been completed.
   bool acquire() {
     if (*init_byte_address == COMPLETE_BIT)
       return true;
     if (*init_byte_address & PENDING_BIT)
-      ABORT_WITH_MESSAGE("__cxa_guard_acquire detected recursive initialization");
+      ABORT_WITH_MESSAGE("__cxa_guard_acquire detected recursive initialization: do you have a function-local static variable whose initialization depends on that function?");
     *init_byte_address = PENDING_BIT;
     return false;
   }
 
   /// The init byte portion of cxa_guard_release.
   void release() { *init_byte_address = COMPLETE_BIT; }
   /// The init byte portion of cxa_guard_abort.
   void abort() { *init_byte_address = UNSET; }

 public:
   /// The init byte portion of cxa_guard_acquire. Returns true if
   /// initialization has already been completed.
   bool acquire() {
     LockGuard g("__cxa_guard_acquire");
     // Check for possible recursive initialization.
     if (has_thread_id_support && (*init_byte_address & PENDING_BIT)) {
       if (*thread_id_address == current_thread_id.get())
-        ABORT_WITH_MESSAGE("__cxa_guard_acquire detected recursive initialization");
+        ABORT_WITH_MESSAGE("__cxa_guard_acquire detected recursive initialization: do you have a function-local static variable whose initialization depends on that function?");
     }
 
     // Wait until the pending bit is not set.
     while (*init_byte_address & PENDING_BIT) {
       *init_byte_address |= WAITING_BIT;
       global_cond.wait(global_mutex);
     }
 
while (true) {
 
       if (last_val == COMPLETE_BIT)
         return true;
 
       if (last_val & PENDING_BIT) {
 
         // Check for recursive initialization
         if (has_thread_id_support && thread_id.load(std::_AO_Relaxed) == current_thread_id.get()) {
-          ABORT_WITH_MESSAGE("__cxa_guard_acquire detected recursive initialization");
+          ABORT_WITH_MESSAGE("__cxa_guard_acquire detected recursive initialization: do you have a function-local static variable whose initialization depends on that function?");
         }
 
         if ((last_val & WAITING_BIT) == 0) {
           // This compare exchange can fail for several reasons
           // (1) another thread finished the whole thing before we got here
           // (2) another thread set the waiting bit we were trying to thread
           // (3) another thread had an exception and failed to finish
           if (!init_byte.compare_exchange(&last_val, PENDING_BIT | WAITING_BIT, std::_AO_Acq_Rel, std::_AO_Release)) {